The invention relates to optical devices and more particularly to photodetectors made as an integrated circuit.
A continuing object of integrated circuit manufacturing is to increase the speed of operation. One of the issues relating to using integrated circuits is the interconnect with the integrated circuit itself. The interconnect itself has and creates speed limitations. Some of these relate to the physical interconnect and others relate to distances that must be covered by the signal that is either received or transmitted by the integrated circuit. One of the techniques that is being studied to improve this is the use of light as opposed to an electrical signal for the source of information for the integrated circuit. The typical integrated circuit has a silicon substrate, which does provide the capability, albeit a not very good one, of being a photodetector. One of the reasons silicon is not considered a particularly good photodetector is that its absorption coefficient is low compared to some other materials such as germanium.
The technique for detecting light using silicon or germanium is to detect carriers generated by the incident light. The incident light must be at frequency that is absorbed by the material as opposed to frequency at which the light is passed. In silicon the frequency of the light that is absorbed has a wavelength less than 1.1 microns, whereas frequencies with a wavelength greater than 1.1 microns are passed. One standard frequency below the 1.1 micron wavelength is the standard for local area networks (LAN), which has a wavelength of 850 nanometers. The light that has a frequency that can be absorbed by the silicon, which generates holes and electrons as the light penetrates and is adsorbed by the silicon. These carriers are then collected to perform the detection of the incident light using biased doped regions in the silicon. The bias is sufficient to fully deplete the substrate or well regions. The incident light carries the information that is to be processed by the integrated circuit.
The efficiency of the detector is increased if more of these carriers, which are generated by the light, can be collected. One of the difficulties with silicon is that about 98% of the carriers are generated over about 20 microns of distance, i.e., the light penetrates into the silicon about 20 microns before it is substantially fully adsorbed. It is difficult to collect most of these carriers, the 98%, rapidly. The electric field provided by the biased doped regions attracts the carriers. As the distance between the doped regions and the carriers increases, the electric field diminishes. These carriers that are in the low electric field areas are too slow in reaching the doped regions where they can be detected. The result is a rate of detection which is not a fast enough to provide a significant improvement over that available by using normal electrical signals.
To have the requisite speed of detection, the collectors of the carriers must be in closer proximity to the generation of the carriers. A technique for improving the speed was to isolate many of the carriers that were generated relatively far from the doped regions using conventional SOI type substrates. Thus, the incident light would generate carriers at the surface and continue generating carriers but most of the carriers would be generated below the insulating layer that is part of an SOI substrate. This improves the speed because only the carriers that were generated close to the electrodes reached the doped regions, but most of the carriers were generated below the insulating layer so that the detection itself was difficult. Detection typically includes biasing doped regions to attract the holes or electrons that are the carriers that are generated by the incident light. The fact that these doped regions are biased inherently makes it difficult to detect very small amounts of charge. Thus, the more charge that is available for detection, the more effective the detection will be.
Thus there is seen a need for a photodetector in a semiconductor that can be fast enough and reliable enough to detect signal information from light.